textproduct: Rapid City

KEY MESSAGES

- Cool and wet this weekend, with daily precipitation in the region.

- Gradual warmup early next week.

UPDATE

Issued at 935 PM MDT Fri Sep 18 2026

The evening update to the forecast utilized latest radar and satellite imagery and latest hi-res model guidance to show a diminishing trend in the PoP field overnight, before PoPS increase again Saturday afternoon into Saturday night.

Latest radar imagery shows a line of rain showers and thundershowers tracking eastward while fading out. Earlier, this line of storms produced wind gusts around 40 to 55 mph, heavy rain, and frequent lightning. Early rainfall reports range mainly from about a quarter inch to half inch with a few isolated stations up to three-quarters of an inch. Rain is still falling so final reports are expected Saturday morning.

Beginning Saturday afternoon, no changes were made to the inherited forecast. Widespread precipitation is expected Saturday into Sunday as the upper low tracks towards the area. Drier and warmer weather is still expected next week.

DISCUSSION

(This Evening Through Friday) Issued at 213 PM MDT Fri Sep 18 2026

Current Water Vapor and upper air models show southwest flow over the forecast area, as upper low digs into the Rockies. 18Z surface analysis depicts a low over southeastern SD, with frontal boundary stretching into central Nebraska. Winds are generally light out of the north, with temperatures in the 60s to upper 70s.

Out ahead of the upper low, a disturbance will lift up into the region this evening. The line of showers/storms is currently in central Wyoming moving northeast, expected to enter the forecast area in a few hours. Generally light precipitation with the evening precipitation, but accumulations nearing a tenth of an inch may be possible in heavier precip cores. Precipitation may linger into the overnight hours, and some showery development is possible again during the day Saturday. There's a low-end chance for a severe storm Saturday afternoon, given some CAPE values 500-1000 j/Kb, however weak effective shear (20-30kts).

Attention then turns to the main event, as the upper low moves over Wyoming later Saturday into Sunday. Widespread precipitation is expected to move into the region over this timeframe. Heaviest amounts are expected Sunday morning and afternoon and will taper off from west to east Sunday night into Monday. LREF probabilities for 24 QPF 0.5" or more has a bullseye of 70-90% over the Black Hills into southwestern SD. Probabilities for 1" or more QPF range from 30-60% in the same area. Expect cooler temperatures as well as the low moves into the plains, with high temperatures Sunday only reaching around 60 over the plains, and the low to mid 50s over the Black Hills.

By the latter half of monday, the pattern changes again as upper ridging builds over the plains, bringing a gradual warming trend Monday through much of next week. Temperatures climb back to at or above normal by Tuesday and Wednesday.

AVIATION

(For the 18Z TAFS Through 18Z Sunday) Issued At 1108 AM MDT Sat Sep 19 2026

Low clouds are expected to stick around through much of the TAF period. Ceilings could see some gradual improvement through the afternoon, however, they should still be limited to no higher than MVFR, especially as showers and thunderstorms redevelop during the afternoon (~19/19-21z) and evening hours. Expect gusty/erratic winds associated with any stronger thunderstorms or heavier precipitation cores. Hail cannot be ruled out with any of the stronger thunderstorms this afternoon/evening. Mist is expected to mix in with the precipitation late tonight into tomorrow reducing the visibility to IFR (possibly LIFR) at KRAP and KGCC.

UNR WATCHES/WARNINGS/ADVISORIES

SD...None. WY...None.